fix(jsx): correctly comment first line of JSX

The first and last lines of JSX should be commented with `// %s`, rather
than `{/* %s */}`. However, the current way of configuring the plugin
does not allow us to check if the current node is a `jsx_element` with a
parent of `parenthesized_expression`. We could allow checking the parent
node in the configuration, but that would be a bit tricky and a lot of
work.

Since this is (hopefully) a 1-time exception, then it seems okay to add
an explicit check for this. If we have similar cases in the future
though, then we should think of making this configurable somehow.

Fixes #29

lua/ts_context_commentstring/internal.lua

@@ -184,8 +184,21 @@ function M.check_node(node, language_config, commentstring_key)
   end
 
   local node_type = node:type()
-  local match = language_config[node_type]
 
+  -- A workaround for an edge case in JSX.
+  -- The first line of JSX should be commented with `// %s` rather than
+  -- `{/* %s */}` but the general logic does not allow us to configure that
+  -- behaviour. This is a bit of an edge case though, and so we have this
+  -- workaround here rather than allowing for more complex configuration.
+  -- For more information, see:
+  -- https://github.com/JoosepAlviste/nvim-ts-context-commentstring/issues/29
+  -- NOTE: We should NOT add any more of these sort of workarounds
+  local is_first_line_of_jsx = node_type == 'jsx_element' and node:parent():type() == 'parenthesized_expression'
+  if is_first_line_of_jsx then
+    return language_config[commentstring_key] or language_config.__default or language_config
+  end
+
+  local match = language_config[node_type]
   if match then
     return match[commentstring_key] or match.__default or match
   end